1. Field of the Invention
This invention relates to photo detectors and has an important application in monitor detectors for optical sources.
2. Related Art
Optical sources such as lasers for use in optical communication systems, for example, usually need to provide optical output power which varies little over time and between nominally identical devices. Typically, to achieve this stability and repeatability, an optical-electrical control loop is employed, in which a small fraction of the optical output of the source is directed onto the active area of a monitor photo-detector mounted in a suitable position relative to the optical source. The monitor converts the incident optical signal into an electrical output signal which is representative of the optical output of the source. This electrical signal is then, in turn, employed to control the optical source. In the case of a semiconductor laser optical source, for example, a common arrangement is to monitor the light emerging from the back facet of the laser and use the resulting electrical signal to control the drive current of the laser.
The monitored fraction of the optical output power depends very sensitively on a number of device and constructional parameters, making it difficult if not impossible to fix the photodiode characteristics in relation to a particular laser device. On the one hand, to alleviate the problems of positioning the photo-detector relative to the optical source the region over which the detector is photo-sensitive should be fairly large. A large photo-sensitive region also provides the required slow response of the monitor device, which ought to respond only to relatively slow changes in the average output, but not to the very rapid changes corresponding to modulation of the output of the optical source. On the other hand, large devices tend to be optically very sensitive, and in many instances prove to be too sensitive. Conventional methods to lower this responsivity by moving the photo-detector towards glancing incidence and off the axis of the monitored light beam, or reducing the size of its photo sensitive region or both, incur penalties in terms of the positional tolerance and the stability with temperature as the far field pattern of the optical source changes.
Further problems with conventional arrangements arise from the fact that the coupling efficiency into a fibre varies widely with the exact fibre lens parameters. The user is usually interested in monitoring the optical power coupled with the fibre rather than the actual power generated by the laser. If the monitor diode is pre-aligned to the laser, differences in coupling efficiency from laser to fibre will cause a large variation in the parameter of interest.